wire customid through api

litellm/llms/vertex_ai/files/transformation.py

@@ -511,7 +511,11 @@ class VertexAIJsonlFilesTransformation(VertexGeminiConfig):
                 litellm_params={},
                 cached_content=None,
             )
-            vertex_jsonl_content.append({"request": vertex_request_body})
+            vertex_line: Dict[str, Any] = {"request": vertex_request_body}
+            custom_id = _openai_jsonl_content.get("custom_id")
+            if custom_id is not None:
+                vertex_line["custom_id"] = custom_id
+            vertex_jsonl_content.append(vertex_line)
         return vertex_jsonl_content
 
     def _get_gcs_object_name(

tests/test_litellm/llms/vertex_ai/files/test_vertex_ai_jsonl_custom_id_preserved.py

@@ -0,0 +1,28 @@
+import json
+
+from litellm.llms.vertex_ai.files.transformation import VertexAIJsonlFilesTransformation
+
+
+def test_should_preserve_custom_id_in_vertex_jsonl_transformation():
+    """Ensure custom_id is preserved in Vertex JSONL batch uploads."""
+    transformer = VertexAIJsonlFilesTransformation()
+    openai_jsonl_content = [
+        {
+            "custom_id": "req-1",
+            "body": {"model": "gemini-2.5-flash-lite", "messages": []},
+        },
+        {
+            "custom_id": "req-2",
+            "body": {"model": "gemini-2.5-flash-lite", "messages": []},
+        },
+    ]
+
+    vertex_jsonl_content = transformer._transform_openai_jsonl_content_to_vertex_ai_jsonl_content(
+        openai_jsonl_content
+    )
+
+    assert [item.get("custom_id") for item in vertex_jsonl_content] == [
+        "req-1",
+        "req-2",
+    ]
+    assert all("request" in item for item in vertex_jsonl_content)